1. Field of the Invention
The invention relates to a method and apparatus for monitoring and controlling rate of bath turnover and has particular though not exclusive application to apparatus for dyeing yarn packages.
2. Description of the Related Art
In the preparation of yarn for the manufacture of textile products, the yarn may be prepared for dyeing by winding it on to a perforated cylindrical core or dye tube formed as a cylindrical bobbin, such a wound bobbin being referred to as a package. A plurality of packages can be stacked on a plurality of slightly smaller diameter perforated tubes, or possibly cruciform or triangular extrusions, called spindles, mounted vertically on a circular manifold or carrier. The carrier, after loading with dye tubes and packages, can be lowered into a cylindrical vessel known as a kier. The kier is equipped with a pump, a heat exchanger and a reversing valve and may be closed and pressurized. Dyeliquor is then circulated through the packages in both of opposite directions to effect dyeing under pressure and at high temperature.
If liquor were to be made to flow in one direction only, then there would be a tendency for dyeing to be uneven since fibres contacted first by dyeliquor are treated with the most concentrated or least exhausted dyeliquor. If the dye is permitted to "strike" on the inside of the package only, it may take a long time for the concentration level to equalize in the mass of the fibre. It will eventually do this by a process called "migration", by which dye in the more concentrated regions tends to move back into the dyeliquor as the concentration of the dyeliquor decreases and thus moves into fibres which have absorbed lower amounts of dye.
As the temperature is the driving force by which many dyestuffs are absorbed into the molecular structure of fibre, raising the temperature on one-way flow, while accelerating the dyeing process, may also increase initial unevenness. It will also increase the need to migrate to a level basis again later. Thus, while dye concentration can eventually become completely even by reliance on migration, the time required for such equilibrium to be reached is many times greater than the time required for a mere application phase; and, it is clearly advantageous to take steps to reduce the need for migration by providing conditions to encourage the dye to strike as evenly as possible.
Periodic flow reversal is an accepted technique to minimize the unevenness; and, optimum evenness can be obtained by relating both frequency of flow reversal and rate of temperature rise to rate of dyebath turnover. Generally speaking, if the flow is reversed as soon as the entire dyebath has passed through the yarn once, in one direction at constant temperature, and then once in the other direction, the difference in depth of shade across the package wall will be more or less as low as possible in the "strike phase". If, as is usual, the process requires that the temperature be increased, then for maximum uniformity of dye application, the amount of the increase needs to be in relation to bath turnover rather than to time.
The physical characteristics of the yarn and consequently the resistance to flow of the package itself may change as temperature is increased, thus altering its resistance to flow, so that the rate of flow may not be the same in both directions and the expression "liters per minute" may be a less meaningful measure of the actual work done on the yarn than "bath turnovers per minute".
The package characteristics may change also as flow rate is increased, with a tendency for some packages to "blow" on inside out flow. This does not always mean that the package is physically destroyed, but that deformation of the package or separation of fibres may take place in some areas and not in others, resulting in unequal treatment in different places in the package, particularly at the top or bottom of the spindle or at package spaces, thereby resulting in unlevel application.
On outside to in flow, the spaces in the interstices of the yarn can be almost closed by excessive pressure such that flow is virtually stopped. Also, any deviation from a tolerance acceptable to the dyer needs to be instantly brought to his notice so that a malfunction of a mechanical component can be detected and corrected before sub-standard products are produced, instead of, as happens all too frequently, such correction being effected as a result of an inquest after sub-standard products are produced. The sensitivity and speed of response of flow rate control devices are of major importance.
It was once standard practice to set flow rate by means of a throttle valve which was normally in a wide open position; to set reversal times by means of a mechanical timer; and in the same way, to set temperature on a clockwork pen type recording instrument with a circular chart.
To give a simplified example, if the temperature is to be raised from say 90.degree. C. to 180.degree. C. in 30 minutes, or 3.degree. C. per minute, at a bath turnover rate of 3 per minute, the most level application of dyestuff will result from the carefully controlled increase of 1.degree. C. per bath turnover. Different dyestuffs strike in different temperature ranges and different combinations of dyestuffs will add complications to this simple example. For optimum results, different rates of temperature increase may be required in different temperature ranges during the dye application period.
It is not only to maintain an existing standard, but also to establish and fully exploit the potential of the apparatus on the widest range of packages, that the rate of dyebath turnover should be monitored and accurately controlled.
Accurate measurement of flow rate in close coupled circulating systems such as packing dyeing machines is however not easy, partly because of the inevitable turbulence and eddy currents that are invariably present and also because of the variations in temperature that are inherent in the process. Also, the higher the velocity in the circulating system the further do the flow characteristics depart from the ideal laminar condition required for accurate flow measurement, and the more difficult it becomes to measure the work done on the yarn.
The work done by the pump can be determined from the current which the drive motor which drives it consumes, but in the majority of pumps used in dyeing machines, this does not bear a linear relationship to liquor circulation. It is conventional practice to use volute type pumps in which the pump casing is so shaped as to modify the pump curve by converting velocity into pressure at a chosen part of its range, and it is not uncommon for the current curve to rise to a maximum and then to fall off again. If there are two different flow rates at which the same current is consumed it is obviously unsuitable for control purposes.